1. Field of the Invention
This invention relates to crystalline complex perovskite compounds which are useful as a material for capacitors or other ceramic materials and more particularly, to a method for producing such complex perovskite compounds.
2. Description of the Prior Art
For the manufacture of oxide ceramic powders, usual practice is to mix metal oxides, carbonates and the like in a suitable milling device such as a ball mill and calcine the mixture at suitable temperatures. The resultant powder is, however, disadvantageous in that it is large and irregular in size, so that sinterability is not good; because of the non-uniformity of the composition, the characteristics of the powder are not consistent; and since the ingredients are mechanically mixed and milled, for example, by a bal mill, incorporation of impurities is inevitable.
In order to avoid the above disadvantages, various preparation processes using starting powders have been proposed. One such process is a process of hydrolyzing metal alkoxides. This process makes use of the reaction between a metal alkoxide of the formula, M(OR)n, in which M is a metal atom having a balance of n, and R represents an alkyl group, and water thereby forming a metal oxide or hydroxide and an alcohol. For instance, BaTiO.sub.3 having a perovskite structure can be prepared by dissolving barium alkoxide and titanium alkoxide at a molar ratio of 1:1 in an organic solvent and dropping the solution into water for hydrolysis. This is described, for example, in Japanese Laid-open patent application No. 57-82119. The powder obtained by the above process has been already crystallized at a low powder formation temperature of 70.degree. to 80.degree. C. Further, the powder has a fine size and good sinterability, and is substantially free of any impurities as will occur at the time of mixing in a ball mill.
Oxides which contain two or more metals and can be prepared as a crystalline powder by similar processes as described above include, for example, perovskite compounds or solid solutions thereof such as SrTiO.sub.3, Ba(Ti.sub.1-x Zr.sub.x)O.sub.3, BaZrO.sub.3, (Ba.sub.1-x Sr.sub.x)TiO.sub.3 and the like as described in Japanese Laid-open patent application No. 58-2220; ferrite compounds such as MnFe.sub.2 O.sub.4, (Mn.sub.1-x Zn.sub.x)Fe.sub.2 O.sub.4, NiFe.sub.2 O.sub.4 and the like as described in Japanese Laid-open Patent Application No. 56-26726; germanic acid salts such as SrGeO.sub.3, PbGeO.sub.3, ZnGeO.sub.3 and the like as described in Japanese Laid-open patent application No. 58-199717; and other oxides such as PbWO.sub.4, SrAs.sub.2 O.sub.6 and the like.
However, the hydrolysis processes of metal alkoxides have a serious problem that starting metal alkoxides are very expensive.
Other processes have also been proposed to obtain a fine powder by reaction of two or more metal salts or hydrolyzates in a strongly alkaline aqueous solution. These processes are useful in preparation of crystalline powders of simple perovskite compounds or solid solutions thereof including, for example, BaTiO.sub.3 (Journal of the Industrial Chemistry, Vol. 71, No. 1 and Japanese Laid-open patent application No. 59-39716), Ba(Ti.sub.1-x Zr.sub.x)O.sub.3 and (Ba.sub.1-x Sr.sub.x)TiO.sub.3 (Japanese Laid-open patent application No. 60-10303), and PbTiO.sub.3 (Japanese Laid-open patent application No. 61-158821).
For the preparation of complex perovskite compounds having a more complicated crystalline structure and represented, for example, by the following formula, EQU A(B.sub.x, C.sub.y)O.sub.3
in which A, B and C are, respectively, metal atoms having valences of p, q and r, and x+y=1 provided that q.noteq.r and xq+yr=6-p, the above-described known processes or techniques are not satisfactory. For instance, with the hdyrolysis method of metal alkoxides, an amorphous powder is formed. According to the process using a metal salt or its hydrolyzate, only four compounds indicated above are now prepared, but crystalline complex perovskite compounds have not been produced by the latter process. Even though the individual compounds of the solid solutions are taken into account, only four types of compounds including BaTiO.sub.3, BaZrO.sub.3, SrTiO.sub.3 and PbTiO.sub.3 are prepared.